1. Field of the Invention
The present invention relates to methods of fabricating thermal ink jet printheads, and particularly to methods of fabricating pagewidth "roofshooter" printheads from an array of silicon wafer subunits (or chips).
2. Description of the Related Art
Generally speaking, drop-on-demand ink jet printing systems can be divided into two types; one type using a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle; or another type using thermal energy to produce a vapor bubble in an ink filled channel that expels a drop.
Thermal ink jet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The printing system may be incorporated in either a carriage type printer or a pagewidth type printer. The carriage type printer generally has a relatively small printhead, containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. For an example of a cartridge type printer, refer to U.S. Pat. No. 4,571,599 to Rezanka. In contrast, the page width printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process. Refer to U.S. Pat. No. 4,463,359 to Ayata et al for an example of pagewidth printing and especially FIGS. 17 and 20 therein.
U.S. Pat. No. 4,463,359 mentioned above discloses a printhead having one or more ink filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle from the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. The current pulses are shaped to prevent the meniscus from breaking up and receding too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet print devices are shown, such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate for the purpose of obtaining a pagewidth printhead. Such arrangements may also be used for different colored inks to enable multi-colored printing.
U.S. Pat. No. 4,789,425 to Drake et al (the disclosure of which is herein incorporated by reference) discloses a thermal ink jet printhead of the type which expels droplets on demand towards a recording medium from nozzles located above and generally parallel with the bubble generating heating elements contained therein. The droplets are propelled from nozzles located in the printhead roof along trajectories that are perpendicular to the heating element surfaces. Such configurations are sometimes referred to as "roofshooter" printheads.
For example, as illustrated in the isomeric view of the printhead 10 in FIG. 1 hereto, arrows 11 depict the trajectory of ink droplet 13 emitted from nozzles 12. The printhead 10 includes a structural member 14 permanently attached to a heater plate or substrate 28 containing an etched opening or feed slot 20 (shown in phantom) which when mated to the structural member 14, forms an ink reservoir or manifold. The cross-sectional view of the printhead 10 in FIG. 2 taken along lines II--II of FIG. 1 illustrates the ink flow path from the feed slot 20 in the heater plate 28 through the nozzles 12 in the roof 24. The ink flows into a channelled recess 18 defined by a cavity wall 22 and channel walls 17 between the roof 24 and heater plate 28, and then passes over a heating element 34 with its addressing electrode 33 and common return 35 before exiting through the nozzle 12. The plan view of the printhead (FIG. 3; taken along lines III--III of FIG. 1) illustrates the recess 18 having four channel walls 17 which produce three ink channels communicating between the nozzles 12 (shown in phantom because they are in the roof 24) and the feed slot 20. (It is understood that a true view along the lines III--III would show a heating element and associated ink channel density of 300 per inch (25 mm) or more, the reduced number being shown here for clarity.)
Drop on demand thermal ink jet printheads as discussed above are fabricated by using silicon wafers and processing technology to make multiple small heater plates and channel plates. This works extremely well for small printheads. However, for large arrays or pagewidth printheads, a monolithic array of ink channels or heater elements cannot be practically fabricated in a single wafer since the maximum commercial wafer size is generally six inches. Even if ten inch wafers were commercially available, it is not clear that a monolithic channel array or heater array would be very feasible. This is because one defective channel or heating element out of 2,550 channels or heating elements would render the entire channel or heater plate useless. This yield problem is aggravated by the fact that the larger the silicon ingot diameter, the more difficult it is to make it defect free. Also, relatively few 81/2 inch channel plates or heater plate arrays could be fabricated in a 10 inch wafer. Most of the wafer would be thrown away, resulting in very high fabrication costs.
To obviate this problem, it is proposed to create a pagewidth printhead by forming an array of roofshooter subunits butted together to form the pagewidth array. However, in order to produce high quality characters with ink jet printers it is essential to provide a printhead with a high density of precisely aligned nozzles so that each subunit in a pagewidth array must be precisely located relative to an adjacent subunit. As can be seen from FIG. 4A which schematically illustrates only the heater plate 28 of FIG. 3 with the heating element 34, electrode 33 and the feed slot 20, in order to provide a high density arrangement of nozzles on a roofshooter pagewidth printhead, the best location for dicing each heater plate (designated a--a and a'--a') intersects the feed slot 20 causing the heater plate to become two separate pieces 28A, 28B (as illustrated in FIG. 4B) which are difficult to realign with each other, or with the roof 24 to construct the roofshooter printhead. One solution to this problem could be to break up the feed slot into a number of smaller slots F.sub.1, F.sub.2, F.sub.3 as shown in FIG. 5. However, the geometry of anisotropic silicon etching causes the slots to be separated by a minimum of 29 mils at the level of the heater elements 34. This amount of separation is unacceptable because it would be difficult to ensure that ink would flow to the heater elements 34' located between the slots since the fluid feed resistance of the heater elements 34' between slots will likely be substantially greater than that of heater elements 34 adjacent to a slot.
Another difficulty in designing a buttable printhead subunit lies in the fact that it is difficult to make electrical connections to the printhead at the same density as the transducer array. For example, it is possible to make thermal ink jet heater and nozzle arrays at a resolution density of 600 elements per inch. However, typical production wire bond densities are limited to about 100 elements per inch. For small arrays, a limited number of heaters can be directly addressed by fanning out the addressing electrode lines to provide for a lower bonding pad density as shown in FIG. 10. However, this technique consumes more silicon area than is required by the transducer array, and it is not possible to use this design with a large continuous array of buttable printhead subunits.